1. Field of the Invention
The present invention relates to a non-reducible dielectric ceramic composition and, more particularly, a non-reducible dielectric ceramic composition used for monolithic ceramic capacitors.
2. Description of the Prior Art
In general, monolithic ceramic capacitors comprises plural dielectric ceramic layers united into a monolithic body, a plurality of internal electrodes formed between adjacent dielectric ceramic layers, and external electrodes formed on opposite sides of said monolithic body and connected to the alternate internal electrodes.
Such monolithic ceramic capacitors are generally produced with a dielectric ceramic composition mainly comprising barium titanate by the steps of preparing green ceramic sheets, applying a paste for internal electrodes on one flat surface of each the green ceramic sheets, stacking and pressing several green sheets to form a green multilayer ceramic body, cutting the multilayer ceramic body into green capacitor chips, firing them in air at a temperature ranging from 1250.degree. to 1350 .degree. C. to form monolithic capacitor chips with internal electrodes, applying a paste for external electrodes on opposite sides of the monolithic capacitor chips, and baking them at a suitable temperature to complete monolithic capacitors.
Accordingly, a material for internal electrodes is required to meet the following conditions: (a) to have a melting point higher than the sintering temperature of the dielectric ceramics as the internal electrodes are fired along with the dielectric ceramic material; and (b) is not oxidized even in an oxidizing atmosphere, and does not react with the dielectric ceramics.
To meet such requirements, noble metals such as platinum, gold, palladium and their alloys have been used as a material for internal electrodes of monolithic ceramic capacitors. However, noble metals are expensive so that use of such noble metals causes increase in the production cost of the monolithic ceramic capacitors. For example, the cost of internal electrodes occupies about 30 to 70% of the production cost of monolithic ceramic capacitors.
Other metals having a high melting point are base metals such as Ni, Fe, Co, W and Mo. However, such base metals are oxidized easily in an oxidizing atmosphere at a high temperature, so that they loose the functions as internal electrodes. Thus, in order to use such a base metal as a material for internal electrodes of monolithic ceramic capacitors, it is required to fire the dielectric ceramic material in a neutral or reducing atmosphere. However, the dielectric ceramic compositions of the prior art are reduced considerably when fired in the reducing atmosphere, resulting in semiconductorization of the ceramic composition.
To solve such problems, it has been proposed in JP-B- S57-42588 to use a dielectric ceramic material comprising a solid solution of a barium titanate system and having a ratio of a barium site to a titanium site greater than the stoichiometric value, i.e., 1.00. Such a dielectric ceramic material is scarcely semiconductorized even if fired in a reducing atmosphere, thus making it possible to manufacture monolithic ceramic capacitors employing a base metal such as nickel as a material for internal electrodes.
On the other hand, the development of electronic techniques have led to considerable miniaturization of electronic devices. For this reason, there is an increasing demand for miniaturization of electronic parts including monolithic ceramic capacitors. It is generally known that the monolithic ceramic capacitors can be miniaturized by use of a dielectric ceramic material with a high dielectric constant or by decreasing a thickness of dielectric ceramic layers. However, the dielectric ceramic materials with a high dielectric constant are large in grain size. Thus, if the thickness of dielectric ceramic layers is decreased to 10 .mu.m or below, the number of crystal grains present in each layer is considerably decreased, resulting in poor reliability of monolithic ceramic capacitors.